The gas-phase ion/molecule reaction of NH2- with SO2F2 proceeds rapidly to yield NSO2- (85%) and HNSO2F- (15%). The NSO2-, a long-sought species, was studied in a number of gas-phase acid-base bracketing experiments by FT-ICR leading to a Delta H-acid(o) of 1381 +/- 21 kJ/mol for HNSO2 at a temperature of 333 K. A lower limit of 3.49 eV has been set for the electron affinity of NSO2.. Ab initio calculations using basis sets corrected in the valence region by the generator coordinate method yield a Delta H-acid(o) of 1386 kJ/mol for HNSO2 at 298 K while the isomeric HOS(O)N is predicted to have a Delta H-acid(o) of 1281 kJ/mol. The electron affinity of the NSO2. radical is calculated to be 3.76 eV. Comparison has also been made with the previously identified NSO- ion for which our calculations predict a proton affinity of 1452 kJ/mol which compares very favorably with the experimentally derived value of 1439 +/- 21 kJ/mol. The optimized molecular structures and harmonic vibrational frequencies obtained at the MP2 level provide a valuable guide toward distinguishing these species.